In conventionally producing twisted strand pairs or quads of insulated conductors, such as those forming telecommunications cable, either a strand supply reel or a strand take-up reel has been revolved about the longitudinal axis of the strands to impart a unidirectional twist. More recently however methods have been devised for forming twisted strand units without the need for revolving the strand supply or take-up by periodically reversing the direction of strand twist imparted to an advancing group of strands. This has become known as S-Z twisting with S referring to left-hand twists and Z referring to right-hand twists. It is usually performed with apparatuses known as accumulators which have mutually spaced twisting heads. Each twisting head normally has one or more sheaves rotatably mounted to a head support which itself is mounted for revolution about the axis of two or more strands advancing side by side through the accumulator.
Heretofore many different types of accumulators have been developed which may be used in producing S-Z twisted strand units. One type is known as a variable storage accumulator, examples of which are disclosed in U.S. Pat. Nos. 3,373,550 and 3,782,092. A variation on this type is known as a variable capacity, in line accumulator, an example of which is shown in U.S. Pat. No. 3,052,079. Another type is termed a fixed storage accumulator which effects S-Z twisting by varying the speed of advance of the strands through the accumulator while maintaining the speed of revolution of the accumulator twisting heads constant about the strands. An example of this is shown in U.S. Pat. No. 3,507,108. A variation of the fixed storage type alters the relative speed of rotation of the twisting heads themselves while maintaining the speed of advance of the strands constant. Representative examples of these accumulators are disclosed in U.S. Pat. Nos. 3,823,536 and 4,006,582.
Most of the described accumulators have been quite complex and weighty with numerous moving parts. Recently, however, as shown in U.S. Pat. No. 3,941,166, yet another type of accumulator has been devised of simplified construction. It employs spaced twister heads mounted for synchronous movement about strands that are advanced at a constant line speed through a hollow tube which extends between the two twister heads. The twister heads and linking tube are driven as a unit in abruptly reversing directions about the strands forming them into S-Z twisted units having periodically spaced points of twist reversal. In operation the tube interior walls rotate with the strands and with the strand reversal points pressed by centrifugal force thereagainst without friction being caused by relative rotary movement between the strands and tube. The spacing between the heads dictates the spacing between the twist reversal points.
As it is functionally desirable to have as few reversal points as possible in any given length of twisted communications strands, it is also desirable to have the spacings and thus the tube itself be as long as possible. The length of the tube however is limited by several restricting factors such as available space for housing the accumulator, the moment of inertia created by abruptly reversing the direction of tube rotation, the various bearings required in supporting the tube, and the friction of the strands created by their progress through the tube. Heretofore there has been developed a method by which the spacing between twist reversal points may be increased without actually increasing the spacing between the twisting heads of an accumulator, an example of which is disclosed in U.S. Pat. No. 3,808,787. With this method, which is herein referred to as "tri-lap", strand units are passed three times between the spaced twister heads of the accumulator which effectively reduces by two thirds the spacing between the two twister heads required for a given spacing between twist reversal points. Conversely, for a given spacing between heads the spacing between twist reversal points is increased three fold.
Unfortunately, it has not been thought feasible to convert accumulators of the type having a tube through which the strand units are passed between twister heads for tri-lap operations. One principle reason for this is the substantial increase that would be created in the contact of twist reversal points with the tube and with the attendant friction thereby created. Furthermore, at least some of the strand units would apparently pass between the spaced twister heads in opposite directions. Were they to contact one another within the tube the twist reversal points would tend to unravel. It therefore would be desirable to devise still another type of accumulator that would posses both the benefits offered by the tri-lap method and the benefits of using a rotatable tube to the ends of which twister heads are mounted.